APRS: Robust Wireless in an Age of IoT
Let's take a look at the basics of APRS and how it fits into a development and prototyping toolkit.
What is APRS?
APRS (Automatic Packet Reporting System) is data over a frequency-modulated bandwidth of 144.390, and has been around for 25 years – it's an early form of the cell phone and internet network structure as we know it today. Much of the structure of APRS can be traced back to Bob Bruninga, who is still working on it. Thanks Bob!
APRS can be delivered through any number of platforms. The most commonly recognized among these is amateur radio on 144.390 Mhz in the US. This radio signal can originate from a hand held transmitter, an Arduino with a radio unit or even a cellphone. There are numerical appends on your callsign to designate what platform you may be operating from. In my most recent case we are flying weather balloons, so my callsign on the balloon is KD0OTI-11 (the -11 indicates a balloon). The append is fairly flexible and it also builds an icon as your reports populate aprs.fi.
The APRS system is a huge network of amateur radio repeaters and digital gateways that are maintained and supported by people like you and me all over the world.
We are all well acquainted with the power of our cell phones, but cell service is bounded by towers, reception and of course, monthly billing. APRS gives a tremendous range of communication with no monthly bill. This got my attention.
What can I do with it?
High-altitude balloons
One of the most common applications of APRS is to transmit location coordinates(GPS). There is a robust stack for formatting your coordinates and getting them out. The tracking on the aprs.fi website is very good and it will give you a fairly accurate prediction based on your current speed and direction. Your waypoints are stored and you can get great data and time stamps from the received packets.
Weather stations
There are a host of resources online to help you build an APRS weather station. Given that Weather Underground has gone behind a paywall, this is a pretty attractive alternative for getting and working with weather data. Also, due to the network of repeaters and the relatively long broadcast distance of two-meter radio, this is a fantastic choice for logging in remote areas.
This project is an ideal classroom exercise and can really get students excited about building their own data logging equipment. The Raspberry Pi is an extremely popular choice and it makes for an ideal use of the early model A and B Pis you may have laying around.
As an extension of this, Google supports an API on the service and it can lead to a huge array of computer science topics and app development projects. When paired with the HX-1, the world is at your doorstep.
Text messages
Because APRS is parsed on the server side, you can send a wide range of data and service formats. SMS or text messages can be sent from any APRS transmitter by replacing the recipient callsign with the domain server address for the recipient and their phone number. Once again, a simple Google search will give you a tremendous amount of options.
As with SMS, the formatting of APRS can be configured to send email as well as text. The gateway domain is formatted with the email address rather than the phone number and, voila, radio to email.
What APRS is not
APRS is a cousin to packet radio, the two share the same formatting AX.25 but the standard frequencies and the modes of application differ.
APRS is also relatively slow, running at 1200 baud, so video, audio and pictures would be better suited for another protocol.
73's
All of this cool stuff happens over the amateur radio (Ham) bands, so you will need a license and some radio gear. I recently bought an inexpensive set of radios to test things with online for about 30 dollars each, so the investment can be relatively painless. I think the time invested in digging into the Technician license is well worth it and there is no Morse Code requirement.
Let us know how you may be using APRS and happy trails.
